Radiation shielding, sometimes known as radiation protection and radiological protection, is the science of protecting people and the environment from the harmful effects of ionizing radiation, which includes both particle radiation and high energy electromagnetic radiation. Ionizing radiation is widely used in industry and medicine, but presents a significant health hazard. It causes microscopic damage to living tissue, resulting in skin burns and radiation sickness at high exposures and statistically elevated risks of cancer, tumors and genetic damage at low exposures. In practice, radiation shielding includes influencing the propagation of radiation in other ways: scattering, collimating, focusing, re-directing, or encapsulating.
It has been argued that it is very difficult to make simple radiation shielding structures because different radiation types interact with condensed matter (solid materials) in a unique ways. Different types of ionizing radiation behave in different ways, therefore different shielding techniques must be used. Particle radiation includes a stream of charged or neutral particles, both charged ions and subatomic elementary particles. This includes solar wind, cosmic radiation, and neutron flux in nuclear reactors. Alpha particles (helium nuclei) are the least penetrating. Even very energetic alpha particles can be stopped by a single sheet of paper. Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite). This is to reduce generation of Bremsstrahlung X-rays. In the case of beta+radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern.
Neutron radiation is not as readily absorbed as charged particle radiation, which makes this type highly penetrating. Neutrons are absorbed by nuclei of atoms in a nuclear reaction. This most-often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next-heavier isotope, many of which are unstable. Cosmic radiation is not a common concern, as the Earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a problem for satellites and astronauts and frequent fliers are also at a slight risk. Cosmic radiation is extremely high energy, and is very penetrating. Electromagnetic radiation includes emissions of electromagnetic waves, the properties of which depend on the wavelength. X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium is used, but lead is much more common; several centimeters are often required. Barium sulfate is used in some applications too. However, when cost is important, almost any material can be used, but it must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water cooled layer of lead on the inside to protect the porous concrete from the coolant inside. The concrete is also made with heavy aggregates, such as Baryte, to aid in the shielding properties of the concrete. Ultraviolet (UV) radiation is ionizing but it is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing, and protective eyewear. Protection from UV is simpler than for the other forms of radiation above, so it is often considered separately. In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily.
Radiation from radioactive isotopes or radionuclides generally consists of high-energy particles or rays emitted during the nuclear decay process. Such radiation generally does not include non-ionizing radiation, such as radio-microwaves, visible, infrared, or ultraviolet light. However, radiation from spontaneous nuclear decay mechanisms can produce alpha particles, beta particles, gamma rays, high energy X-rays, neutrons, high-speed electrons, high-speed protons, and other particles, which are capable of producing ions. Among these emissions, gamma and high energy X-ray radiation are the most common forms of hazardous radiation to which biological organisms, sensitive electronics, etc. are exposed (whether the radiation is manmade or naturally occurring), and therefore most commonly require unique and efficient shielding solutions.
It is well-known that the effectiveness of atomic shielding mechanisms for gamma rays and high energy X-rays is dependent on the atomic number and the density of the shielding material. A denser shielding material with a higher atomic number is a better shielding material for high energy X-rays and gamma rays. For example, lead is heavier than roughly 80 percent of the elements in the periodic table and has a high atomic number; and therefore is the primary material used in most radiation shielding structures. Although, there are other elements with higher densities, such as tantalum and tungsten, lead is chosen because it is readily available, easily fabricated, and has a lower cost.
In the past high energy (ionizing) radiation shielding structures have generally been large-sized structures such as buildings and bulk containers that can be used to house the radiation source. As a result stringent demands have not been placed on the materials' structural properties other than the materials' general effectiveness for radiation shielding. Therefore, concrete and lead have been acceptable materials for constructing such large-scale structures.
However, conventional radiation shielding structures made of lead and concrete are inadequate for the increasingly sophisticated uses of high energy radiation found in some processes and applications. For example, the use of radiation in areas such as medical treatment, and food sterilization requires radiation shielding structures with similar or better performance characteristics than traditional concrete and lead, but made of high-performance high-strength materials. In addition, in some of these applications it is desired to direct radiation into highly localized regions, as in brachytherapy. These structures need to be highly compact and slender, while also requiring high structural integrity and high effectiveness for radiation shielding. Moreover, new radiation shielding structures incorporating moving parts, or having resistance to corrosive environments, or that are bio-compatible, or that have high structural integrity in complex shapes are needed in order to proliferate the use of radioactive radiation in these diverse applications. For example, radiation-shielding structures can take an infinite variety of different shapes and sizes, such as canisters, enclosures, frames, moving parts in various structures and machinery equipment. Ideally, the shielding structure is a topologically continuous uniform structure. However, in order to perform various functions, such as injecting measured doses of radiation in certain directions or in a device with moving parts, the radiation shielding structure may only partly enclose the radioactive source or may have one or more components for performing peripheral functions. For example, a load lock device for a radioactive container may require frequent opening and closing and therefore, the structure may comprise several moving parts and frames. Generally, any such radiation shielding structure or its component still must attenuate the radiation to levels below a maximum allowable level to provide sufficient shielding protection external to the radioactive source. In another form, the radiation shielding structures can be used as a marker in radiography which preferentially blocks the path of radiation, such as imaging and locating orthopedic devices (stents etc.) in the body or locating tumors in Proton Beam Therapy. In this case, the radiography marker is desired to be highly biocompatible.
The main disadvantage of radiation shielding constructions made of lead is its toxicity and limited structural integrity. In contrast, typical engineering materials used in structures and machinery equipment such as steel, aluminum, and titanium do not have good shielding effectiveness and tend to be bulky. Applying other ordinary alloys to radiation shielding applications also has drawbacks. For example, tantalum is both low in mechanical strength and very expensive. Tungsten, on the other hand has higher strength, but is very difficult to fabricate into intricate shapes. Tungsten impregnated plastic has been developed for its formability and reduction in cost, however, its shielding effectiveness is significantly reduced compared to pure tungsten. Furthermore, plastics generally don't have adequate strength and therefore, compact and slender designs cannot be readily obtained. Plastics are also susceptible to environmental degradation.
Accordingly, there is a need to develop new radiation shielding structures providing effective radiation shielding that are corrosion resistant, bio-compatible, and can be formed into designs that are slender and compact with high structural integrity and durability.
A proposed solution according to embodiments herein for radiation shielding structure is to use bulk-solidifying amorphous alloys for radiation shielding. Bulk-solidifying amorphous alloys, or bulk metallic glasses (“BMG”), are a recently developed class of metallic materials. These alloys may be solidified and cooled at relatively slow rates, and they retain the amorphous, non-crystalline (i.e., glassy) state at room temperature. Amorphous alloys have many superior properties than their crystalline counterparts. However, if the cooling rate is not sufficiently high, crystals may form inside the alloy during cooling, so that the benefits of the amorphous state can be lost. For example, one challenge with the fabrication of bulk amorphous alloy parts is partial crystallization of the parts due to either slow cooling or impurities in the raw alloy material. As a high degree of amorphicity (and, conversely, a low degree of crystallinity) is desirable in BMG parts, there is a need to develop methods for casting BMG parts having controlled amount of amorphicity.